This application is based on Japanese Patent Applications Nos. 2000-068591 and 2000-187128 filed respectively on Mar. 8, 2000 and Jun. 22, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image-sensing device incorporating a solid-state image sensor that outputs an electric signal natural-logarithmically proportional to the amount of incident light, and particularly to an image-sensing device that achieves adjustment of white balance by processing the electric signal output from a solid-state image sensor.
2. Description of the Prior Art
The assignee of the present invention once proposed a solid-state image sensor that outputs an electric signal natural-logarithmically proportional to the amount of incident light and that thus offers a wide dynamic range (see U.S. Pat. No. 4,973,833). This solid-state image sensor is provided with a photosensitive means that produces a photocurrent proportional to the amount of incident light, a MOS transistor to which the photocurrent is fed, and a bias means that biases the MOS transistor in such a way that a subthreshold current flows through the MOS transistor. The configuration of the photoelectric conversion circuit formed within each pixel provided in such a solid-state image sensor is shown in FIG. 16.
As shown in FIG. 16, each pixel has a photoelectric conversion circuit composed of a PN-junction photodiode PD that functions as a photosensitive means and a MOS transistor T100 that has its drain and gate connected to the anode of the photodiode PD. A direct-current voltage VPD is applied to the cathode of the photodiode PD and a direct-current voltage VPS is applied to the MOS transistor T100 so that the MOS transistor T100 is so biased as to operate in a subthreshold region.
This photoelectric conversion circuit, composed of a photodiode PD and a MOS transistor T100 as described above, outputs the gate voltage of the MOS transistor T100 as an electric signal logarithmically proportional to the amount of light incident on the photodiode PD. This electric signal, obtained as the gate voltage of the MOS transistor T100, is then amplified by an amplifier circuit provided as the stage succeeding the photoelectric conversion circuit within each pixel, and is then fed out from the pixel. This output signal from each pixel is logarithmically proportional to the amount of incident light, and thus offers the advantage of covering a wide dynamic range.
Here, the electric signal output from the photoelectric conversion circuit shown in FIG. 16 (i.e. the gate voltage of the MOS transistor T100) is given by
Vg=VPS+Vt+(nkT/q)xc2x7ln(Ip/Id)xe2x80x83xe2x80x83(1)
where Vg represents the gate voltage of the MOS transistor T100; Vt represents the threshold voltage of the MOS transistor T100; n represents a constant determined by the gate insulating film capacitance and the depletion layer capacitance; k represents the Boltzmann constant; q represents the amount of electric charge carried by an electron; Ip represents the photocurrent flowing out of the photodiode PD; Id represents the drain current of the MOS transistor T100; and T represents the absolute temperature.
As will be clear from equation (1) above, the electric signal output from the photoelectric conversion circuit is influenced by temperature. Therefore, if this electric signal is simply amplified with an amplifier circuit to produce an output signal that is fed out intact as an image signal, quite inconveniently, the level of the thus obtained image signal varies with the ambient temperature around the image-sensing device.
Moreover, when image sensing is performed with an image-sensing device incorporating such a solid-state image sensor, the electric signal output from the image-sensing device includes a temperature-dependent component, and is thus influenced by the ambient temperature inside the image-sensing device. On the other hand, as the color temperature of a subject to be sensed varies, the wavelength spectrum of the subject varies, and therefore the levels of the color signals, i.e. the R (red), G (green), and B (blue) signals, obtained through an RGB filter vary according to the environment in which the subject is situated. For these reasons, even if a white subject, which is supposed to yield maximum levels in all the R, G, and B signals, is sensed, the image reproduced from the R, G, and B signals obtained as a result of the image sensing does not always appear quite white.
An object of the present invention is to provide an image-sensing device that outputs an electric signal logarithmically proportional to the amount of incident light and that achieves adjustment of white balance by detecting the levels of individual color signals, comparing them with one another, and varying the level of the individual color signals according to the comparison results.
Another object of the present invention is to provide an image-sensing device that can suppress temperature-dependent variation of the output signal thereof.
To achieve the above objects, according to one aspect of the present invention, an image-sensing device is provided with: a photoelectric conversion portion that outputs an electric signal natural-logarithmically proportional to the amount of incident light; and an output circuit that includes a temperature sensor and that corrects the electric signal output from the photoelectric converter on the basis of ambient temperature detected by the temperature sensor.